Transdermal delivery of drugs (medications) has been around for a number of years. Transdermal delivery of drugs offers many advantages over more traditional routes of medicating a patient, i.e. oral administration of the medication or injection methods. One of the advantages of transdermal administration is that it avoids the negative side effects of oral administration, for example irritation of the gastrointestinal tract (some medicines cause ulcers and gastrointestinal bleeding). Transdermal administration also avoids hepatic and systemic toxicity that can result from the necessarily larger doses of the drag that have to be administered orally in order to compensate for hepatic first pass metabolism of drugs absorbed through the gastrointestinal tract, and to maintain therapeutic blood levels of the drug over the dosing interval. Moreover, transdermal delivery offers a painless, convenient, and non-invasive way to administer drugs, providing more stable blood levels with less frequent dosing because of controlled release of the medication. Unfortunately, without altering the skin's barrier these benefits can only be achieved for a few drugs that are able to cross the barrier established by the skin, such as some cardiac drugs and hormones.
The stratum corneum (composed of a lipid bilayers located between dead keratinocytes cells) provides the main resistance to drug delivery through the skin, limiting the transdermal route to drugs having a low molecular weight, i.e. less than 500 Daltons, and having the appropriate lipophilicity. Typically, drugs suitable for transdermal delivery are non-ionic, and/or have high potency, such that therapeutic effects can be achieved with small amounts of transdermal absorption. Other factors affecting absorption are the site of application, length of application, the addition of an absorption vehicle (penetration enhancer or enhancers), such as organic solvents like dimethyl sulfoxide (DMSO), and the integrity of the epidermal barrier. Transdermal delivery of drugs having a molecular weight of greater than 500 Daltons requires some type of skin permeation enhancement mechanism, such as chemical penetration enhancers. Therefore, transdermal administration of drugs has generally been limited to drugs having the required physiochemical properties allowing absorption through the epidermal layer.
Improvements in the transdermal delivery of drugs are sought, such as improvements that may lead to the applicability of transdermal delivery of drags to a broad group of medicines and higher molecular weight biomolecules.